1. Field of the Invention
This invention relates to a method of making compositions of matter for use as a glassy matrix for sealing materials in gas-tight structures of solid oxide fuel cells and to electrochemical assemblies made thereby.
2. Background Art
Fuel cells have attracted interest because they can potentially operate at high efficiencies in converting chemical energy to electrical energy, since they are not subject to the Carnot cycle limitations of internal combustion engines.
One type of fuel cell that is especially appropriate for converting hydrocarbon-derived fuels to electricity is the solid oxide fuel cell (SOFC). A SOFC system includes a cathode, an electrolyte, and an anode. The cathode typically is a porous, strontium-doped lanthanum manganite (LSM) electronically-conducting ceramic; the electrolyte typically is a dense, yttria-stabilized zirconia (YSZ) oxygen ion conducting ceramic; and the anode is typically a porous, nickel-YSZ cermet. Fuel is provided to the anode and air is provided to the cathode. Because electrons cannot move through the YSZ electrolyte, those electrons can be forced to do useful electrical work in an external circuit as oxygen ions formed at the cathode move through the YSZ to react with the fuel at the anode.
An SOFC is able to use as fuels molecules that contain carbon, rather than the highly purified hydrogen required for present-day proton exchange membrane fuel cells. The SOFC-type of fuel cell typically uses a fuel that is natural gas or a synthetic fuel gas containing hydrogen, carbon monoxide, and methane, separated by the electrolyte and its seals from an oxidant such as ambient air or oxygen. With the proper anodes, a SOFC can also use octane and synthetic diesel fuels directly as vaporized. This makes the SOFC adaptable for use as an auxiliary power unit (APU) in vehicles to help meet the growing demand for on-board electrical power.
In SOFCs, hydrogen and carbon monoxide fuels, for example, react chemically with oxygen ions that have passed through the solid electrolyte to produce electrical energy, water vapor, and heat. Even with thin membranes (e.g., 10 micrometers thick) of the YSZ electrolyte, it is necessary to operate the cell at an elevated temperature to keep the internal cell resistance sufficiently low that adequate power can be produced in the external circuit. Consequently, temperatures in the operating SOFC cell may range from 500° to 1100° C. In turn, the seals which keep the fuel and oxidant gas flows separate must be able to function at those elevated temperatures.
Automotive SOFC needs differ from stationary power generation and other fuel cell applications. Due to the limited space available in a vehicle, automotive applications of fuel cells require high volumetric power densities, in addition to the high chemical-to-electrical conversion efficiency that has been established in stationary SOFCs. Just as gasoline and diesel fuels are preferred for their compact storage of great amounts of energy as room-temperature-liquid hydrocarbons, the vehicular fuel cell preferably performs its operation within only a small volume.
In planar SOFCs with high volumetric power densities, gas-tight seals must be formed along the edges of each cell, between each successive cell in a stack, and at the respective gas flow manifolds. An effective sealant creates a gas-tight seal to the cell and stack components, while holding the cell and stack together when exposed to the high temperatures and the reducing and oxidizing gases present in such cells. To realize such planar designs for automotive use, a need remains to find sealants whose performance can withstand the elevated temperatures with both reducing and oxidizing gases in the operating environment of a SOFC, and with the chemical potential gradients that are formed in making the seal between the two gas flows.
In tubular SOFCs for large-scale power plants, at present, seals are made of polymeric elastomer materials, which must be kept at relatively low temperatures (below 150° C.). Consequently, portions of the ionic-conducting tubes are intentionally left electrically inactive to allow for a temperature transition zone to reach down to the temperatures required by the compliant low temperature seals. Not only does this approach result in lower volumetric power densities, but also such added tube length decreases the ability to accommodate the vibrations that are encountered in typical automotive use. High temperature-capable sealing systems can contribute to the desired high power volumetric densities (and also to a lowered mass) by eliminating much of the non-electrically active tube length. Such shortening also will decrease the internal electrical resistance that is associated with the transition lengths needed to protect seals made with existing technology, which can only be used at lower temperatures.
Thus, both planar and tubular designs can benefit in power density from designs which incorporate well-suited high temperature sealing materials.
The benefits to high power density from sealing glasses, as described above, also extend to related electrochemical devices, such as steam reformers and NOx-removing electro catalyst systems. If a NOx reforming system is to be used on a vehicle, it should be of low weight and compact size, so that it can benefit from a high temperature sealant that produces high power density in a SOFC. Differences exist from those of the SOFC in each case. In the case of the NOx reformer, electrical power is applied to the cell by thermoelectric conversion of a temperature gradient from exhaust heat to ambient or by a current imposed from an external circuit, rather than by electricity being produced from the conversion of chemical energy to electrical energy, as in a SOFC. For non-vehicular applications of the fuel cell and NOx devices, and others such as the steam reformer and oxygen electrolysis, it may be desired for other reasons to have a more compact unit operation: there may be only limited retrofit space in a modularized chemical production plant, or there may be a need for portability, as in an oxygen generating medical cart or remote battery charger. In each instance, the sealing material affects whether the design achieves a high power density within applicable space constraints.
A second difference in SOFC requirements for automotive applications is the need for highly efficient conversion to electrical energy in a single or minimum number of processing steps. In contrast, SOFCs intended for use in residential fuel cell co-generation systems can tolerate allowing fuel gas residues (which have not been converted to electricity) to escape from the edges of radial flow plates or the ends of incompletely sealed tube joints, because in such co-generation systems the lost electrical conversion can be used beneficially to generate more of the co-generated heat. Leaky, pressed powder seals such as the talc seals in spark plug insulator compression seals may be suitable for stationary, residential-type co-generation systems. Such seals are less appropriate for automotive use because of their lower efficiency in converting chemical energy to electrical energy.
In view of the automotive and portable power demands for fuel cells operating directly with hydrocarbon fuels, high power density, and high chemical-to-electrical efficiency, the need arises to make compositions for gas-impermeable seals that are suitable for use at the high operating temperatures of SOFCs and their associated structures. Ideally, such seals would exhibit nearly the particular, high thermal expansion coefficient (CTE) that ensures dimensional compatibility among the yttria-stabilized zirconia (YSZ) in the electrolyte, the electrodes, the current collectors, and the structural members.
The prior art includes a publication by N. Lahl, et al., “Aluminosilicate Glass Ceramics As Sealant In SOFC Stacks,” SOLID OXIDE FUEL CELLS VI, S. C. Singhal, et al., editors, PV 99-19, p. 1057–66, THE ELECTROCHEMICAL SOCIETY PROCEEDINGS SERIES, Pennington, N.J. (1999). That publication is incorporated herein by reference. It discloses a glass composition identified as “BAS” that has 45 mol percent BaO; 45% SiO2; 5% Al2O3; and 5% B2O3, with no MgO present. It i that the high BaO content (45%) is needed to attain a relatively high coefficient of thermal expansion. As a result of having so much of the heavy alkaline earth oxide (BaO) in the composition, the estimated thermal conductivity is lowered and environmental stability toward H2O and CO2 is lowered. Although the material composition is alkali oxide-free, the composition is not boric acid-free, because it includes 5% B2O3. The composition is therefore subject to concerns about vaporizing, depositing, and insulating to reduce performance and shorten useful life.
The Lahl, et al. reference discloses that “Glass ceramics [are] formed by controlled crystallization from glass . . . ” Id., p. 1057. Glass ceramics are contrasted with remaining glasses in the next sentence: “As compared to glasses, . . . [glass ceramics] show superior mechanical properties . . . ” Id.
Such difficulties with seals have possibly led to decreased interest in planar cells. The high power densities of planar designs are not as critically needed for the power plant and residential heating applications as they are for vehicular applications.
Related disclosures in the art of preparing SOFCs include U.S. Pat. Nos. 6,099,985 (issued Aug. 8, 2000); and 4,827,606 (issued May 9, 1989), the disclosures of which are also incorporated herein by reference.